Favorite... probably Marbelara. Even though it's just another nature planet with temples (Slugion, Turtemple, Exoticus, Stonar, etc), it's really cool, especially with the addition of the lava stuff. It looks cool from outer space, too. The only real problem is that it's a rip off of Marble Zone.
Least favorite... maybe Slugion. The design was interesting, but not enough was done with it. Besides, having Drageens so soon after Dragoons (or was it the other way around?) is just kind of weird.